A number of prior art processes have been proposed for the preparation of unsaturated carboxylic acids from their corresponding unsaturated aldehydes. Various catalytic vapor phase processes have been described employing complex catalyst systems. Liquid phase reactions include the use of hydroperoxides, molecular oxygen including air in the presence of various solvents and various organic and inorganic metal compounds of silver, nickel, cobalt, manganese, copper, chromium and vanadium used singly, in combination and with other materials such as bromine compounds and chelates. Generally the production of unsaturated aliphatic aldehydes in the liquid phase has been difficult due to polymerization of the unsaturated acids when formed and the co-production of various undesirable peroxides of the unsaturated acids and aldehydes formed during oxidation, resulting in low selectivity to and yield of the unsaturated acids.
To date no commercially successful process has been developed for the liquid phase preparation of unsaturated carboxylic acids, such as methacrylic acid, involving the in situ decomposition of the undesirable peroxidic by products formed during the autoxidation of the aldehyde such as methacrolein. None of the above noted prior art catalysts successfully decompose the peroxidic compounds in situ. The present process is directed to an improved process for the preparation of unsaturated carboxylic acids in high yields and avoiding problems associated with the prior art processes. More particularly, the process relates to the synthesis of unsaturated carboxylic acids by the liquid phase solvent solution oxidiation of the corresponding unsaturated aldehyde and the in situ conversion (decomposition) of unsaturated peroxide compounds formed during said oxidation, by reacting the unsaturated aldehyde, and oxygen in a suitable solvent or mixture of solvents under moderate temperature and pressure conditions in the presence of a catalytic quantity of a zinc salt compound.
U.S. Pat. No. 3,114,769 in an attempt to prevent polymeric by-products, describes a liquid phase process for the oxidation of methacrolein or acrolein to the corresponding acid and peroxide compounds in the presence of molecular oxygen and a small quantity of iodine. The products of the oxidation contained mixtures of unsaturated acids, and large amounts of both acid and aldehyde peroxides and unreacted aldehydes. After separation the peroxide products were separately decomposed to the acid by the catalytic effect of a protonic acid such as p-toluene sulfonic acid and an alcohol forming a hot solvent solution as is further described in U.S. Pat. No. 3,253,025.
In an article by William F. Brill and Fred Lister, Journal of Organic Chemistry, Vol. 26, pp. 565-569, 1961 the metal-salt catalyzed oxidation of methacrolein in acetic acid is described. The methacrolein goes to peroxide products, acid and major amounts of soluble polymer.
The .alpha., .beta.-unsaturated acid products obtained by the process of this invention have many known commercial uses, particularly for the preparation of esters such as methyl methacrylate or methyl acrylate and as monomers for polymer formation.
A particular advantage of the process of the present invention is the discovery that catalyatic amounts of organic and inorganic zinc salt compounds permit the intermediate peroxy compounds formed during oxidation of the unsaturated aldehydes, e.g., permethacrylic acid and methacrolein monopermethacrylate, which are formed during oxidation of methacrolein, to decompose or be converted to methacrylic acid or in the case of other aldehydes the corresponding acids, and thus substantially increase selectivity to and yield of the particular unsaturated acid. Another advantage is that only small amounts of the zinc compounds need be employed and much less peroxy (peroxide) compounds which would otherwise be formed need be removed from the reaction product and further processed thus providing a commercially attractive liquid phase process.